MB-7629 Integrate native view compactor

Invoke native view compactor program, couch_view_group_compactor
to perform btree compactions.

During a test run (on Mac/SSD) with 7M documents and 3 simple views,
time taken for view compaction before and after native compactor is
as follows:

Before:
86.739063s

After:
49.133651s

Change-Id: I8c7526a5f2c46423e370d0823d34d2311d0ce14b
Reviewed-on: http://review.couchbase.org/32309
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>